In light of the virtually worldwide endeavour to reduce the emission of greenhouse gases into the atmosphere, not least as laid down in what is known as the Kyoto protocol, the emission of greenhouse gases which is to be expected in 2010 is to be reduced to the same level as in 1990. The implementation of this plan requires great effort, particularly to reduce the contribution made by CO2 releases caused by mankind. About one third of the CO2 released into the atmosphere by mankind is attributable to energy generation in which mostly fossil fuels are burnt in power plants in order to generate electricity. Particularly due to the use of modern technologies and because of additional political framework conditions, a considerable potential for savings to avoid a further increasing emission of CO2 can be seen to be achieved in the energy-generating sector.
One possibility, known per se and technically manageable, for reducing the CO2 emission in combustion power stations is to extract carbon from the fuels to be burnt, which is implemented even before these are introduced into the combustion chamber. This presupposes corresponding fuel pretreatments involving, for example, the partial oxidation of the fuel with oxygen and/or a pretreatment of the fuel with steam. Fuels pretreated in this way mostly have a high fraction of H2 and CO and, depending on the mixture ratios, have calorific values which, as a rule, lie below those of natural gas. Depending on their calorific value, gases produced synthetically in this way are designated as Mbtu or Lbtu gases which are not readily suitable for use in conventional burners designed for the combustion of natural gases, such as may be gathered, for example, from EP 0 321 809 B1, EP 0 780 629 A2, WO 93/17279 and EP 1 070 915 A1. All the above publications, which are incorporated by reference as if fully set forth, describe burners of the fuel premixing type in which in each case a swirl flow consisting of combustion air and of admixed fuel is generated, which widens conically in the flow direction and which in the flow direction, after emerging from the burner, becomes unstable due to the increasing swirl, as far as possible after a homogeneous air/fuel mixture is obtained, and changes to an annular swirl flow with backflow in the core.
Depending on the burner concept and as a function of the burner power, liquid and/or gaseous fuel is introduced to the swirl flow forming inside the premix burner, in order to produce as homogeneous a fuel/air mixture as possible. As mentioned above, however, it is appropriate, for the purposes of a reduced pollutant, in particular CO2, emission, to employ synthetically treated gaseous fuels alternatively to or in combination with the combustion of conventional types of fuel, and therefore special requirements arise with regard to the structural design of conventional premix burner systems. Thus, synthesis gases, in order to be fed into burner systems, require a multiple fuel volume flow, as compared with comparable burners operated with natural gas, thus resulting in markedly different flow impulse behavior. On account of the high fraction of hydrogen in the synthesis gas and the associated low ignition temperature and high flame velocity of the hydrogen, there is a high tendency of the fuel to react which leads to an increased risk of flashback. In order to avoid this, it is appropriate as far as possible to reduce the average staying time of ignitable fuel/air mixture within the burner.
A method and a burner for the combustion of gaseous or liquid fuel and of fuel containing hydrogen or consisting of hydrogen, synthesis gas in brief, have become known, as described in WO 2006/058843 A1. In this case, a premix burner, which has also become known as a double cone burner, with a downstream mixing zone according to EP 0 780 629 A2 is used, which is illustrated diagrammatically in a longitudinal sectional illustration in FIGS. 2a and b. The premix burner arrangement provides a swirl generator 1 which widens conically in the burner longitudinal axis and which is delimited by swirl producing shells 2. Means for the infeed of fuel are provided axially and coaxially around the burner axis A of the swirl generator 1. Thus, liquid fuel Bfl passes into the swirl space through an injection nozzle 3 positioned along the burner axis A at the location of the smallest inside diameter of the swirl generator 1. Along tangential air inlet slots 4, via which combustion air L enters the swirl space in a tangential flow direction, gaseous fuel Bg, preferably natural gas, is admixed to the combustion air L. In addition, injection devices 5 are provided (see FIG. 2b) which serve for the further infeed of synthesis gas BH2.
The fuel/air mixture forming within the swirl generator 1 passes as a swirl flow through a transitional portion 6, which provides flow means 7 stabilizing the swirl flow, into a mixing pipe 8 in which a fully homogeneous intermixing of the fuel/air mixture forming takes place, before the ignitable fuel/air mixture is ignited within a combustion chamber B following the mixing pipe 8 downstream. On account of a discontinuous enlargement of the flow cross section during the transition from the mixing pipe 8 into the combustion chamber B, the swirl flow of the intermixed fuel/air mixture breaks open, at the same time producing a recirculation flow RB in the form of a backflow bubble in which a spatially stable flame front is established.
The flow profile forming along the burner is illustrated in FIG. 2a and is distinguished by a marked velocity maximum longitudinally with respect to the burner axis A, the amount of which lies mostly three to four times above those flow velocities which can be formed near the burner wall. On account of this drastic velocity difference between the burner axis and burner wall, local flow vortices are established near the burner wall, which lead to local fuel concentrations and, particularly in the case of an additional infeed of synthesis gas, contribute, because of the high ignition potential caused by the hydrogen fraction, to an increased risk of flame flashback which it is appropriate to avoid. In order to reduce the risk of flame flashback, therefore, along the mixing pipe film hole orifices, known per se, are provided, via which supply air is fed in along the inner wall of the mixing pipe in order to form a near-wall air film.
In order to prevent the hydrogen-containing synthesis gas reaching regions near the burner wall, according to the diagrammatic longitudinal sectional illustration in FIG. 2b the synthesis gas BH2 is discharged into the swirl space of the swirl generator 1 at about 60° to the burner longitudinal axis A. In particular, hydrogen-rich fuels with hydrogen fractions of >50% typically have very high flame velocities and, furthermore, have a very much lower volume-specific calorific value (MJ/m3) and therefore very much larger quantities of hydrogen-containing fuel are required which have to be supplied to the burner in order to achieve a desired power-related combustion heat. Thus, in particular, it is shown in what are known as high-pressure tests that, even in the swirl space or along the mixing zone of the burner, ignition phenomena arise which are attributable to an insufficient intermixing of the hydrogen-containing fuel fed with a large volume flow into the burner. Even in cases where no flashback phenomena occur, an insufficient mixing of the hydrogen-containing synthesis gas and the combustion air ensures a diffusion-like combustion which ultimately leads to increased nitrogen oxide emissions. There is therefore the desire to conform to the requirements for the avoidance of flashback phenomena and to the NOx emission limits demanded in light of increasingly more stringent environmental requirements.
The disadvantages which the premix burner concept known hitherto entails are summarized below in inconclusive form,
1. There are inadequate precautions for the avoidance of flame flashback events which are attributable, inter alia, to insufficient flow coordination between the hydrogen-containing fuel stream to be fed into the burner space and the fuel/air swirl flow forming within the swirl generator.
2. Increased NOx emissions which occur as a result of an additional fuel enrichment of synthesis gas along the burner axis and of an accompanying temperature rise.
3. A complicated form of construction of the burner arrangement on account of a multiplicity of fuel lines which lead into the swirl space and are fed in each case via separate fuel distributor circuits which, overall, also cause an insufficient flow coordination referred to above.
4. The power variation of the burner due to the variation in the fuel supply is very limited, especially since fuel instabilities are formed which are distinguished, inter alia, by the occurrence of combustion chamber pulsations.